1. Field of the Invention
The present invention relates to safety devices, and particularly to master shields for power take-off shafts such as those mounted at the back of an agricultural vehicle with a 3-point hitch.
2. Description of the Related Art
Work vehicles such as agricultural and industrial tractors frequently have a power take-off shaft (PTO) extending therefrom to provide power to implements or other devices outside the vehicle. These PTO shafts typically rotate at hundreds of revolutions per minute and can be quite dangerous if left exposed. It therefore has become the nearly universal practice to provide a U-shaped shield around the PTO shaft, e.g., as shown in U.S. Pat. No. 2,443,035 (Hardy). The shield cannot be too close to the PTO shaft, since it also is common practice to provide shielding around the shaft being driven by the PTO, e.g., as taught in U.S. Pat. No. 3,462,975 (Skromme et al.), and adequate space must be provided within the U-shaped master shield for the implement shielding.
One effect of having a master shield over the PTO is that it may increase the difficulty of linking an implement drive shaft to the PTO. An early response to this situation was simple to make the PTO shield detachable, e.g., as taught in U.S. Pat. No. 3,389,763 (Meinert). However, some people have detached the safety shield and simply left it off, leaving an unprotected PTO. Pivotally mounted master safety shields then were developed, e.g., as taught in U.S. Pat. No. Re. 30,152 (Davis).
This flip-up shield also has certain disadvantages, however. Many work vehicles are provided with a 3-point hitch positioned around the PTO shaft. The center link of the 3-point hitch typically is positioned directly above the PTO master shield. If the hitch is lowered too far, the center link may hit and damage the PTO master shield. U.S. Pat. No. 4,553,950 (TEICH) teaches a PTO master shield designed to reduce this possibility by pivoting not only up from a stable position, but down as well.
Even this design still has the disadvantage that it depends upon the operator for proper function. The operator may neglect to put the safety shield down before lowering the center link, in which case the center link will still hit and damage the master shield. If the shield never gets repaired or replaced, the vehicle again is operating without the benefit of a proper PTO master safety shield.
In addition, the operator might flip the PTO shield up while connecting the PTO, and then neglect to put it back in its proper position before operating the machine. The center link then is very likely to hit the master shield, again damaging it.